1. Field of the Invention
The present invention relates to a cushion pin with an elastic member, a wear plate and a load supporting device; to a die cushion for equalizing a cushion pressure by using two or more of them; and further to a press machine having the die cushion. The invention also relates to a pressing method.
2. Description of the Related Art
When the die cushion is used to perform pressing (especially, deep drawing), it is necessary to keep constant a work's blank holding force (cushion pressure) by eliminating poor accuracy (inclination) of a blank holder, variations in length of cushion pins, parallelism error of a die cushion pad for holding the cushion pins, deflection or inclination of a slide of a press machine, and the like. Therefore, a cushion pin pressure-equalizing device is used conventionally. Such a cushion pin pressure-equalizing device is described in, for example, Patent Document 1 (Japanese Utility Model Publication No. 5-27215, pp. 1-2, FIG. 4).
FIG. 15 is a sectional diagram of a press machine using a conventional cushion-equalizing device, showing an example of a cushion pin pressure-equalizing device described in the Patent Document 1. This prior art will be described with reference to FIG. 15.
In FIG. 15, an upper press die 51 is fixed to a slide 52 of the press machine, and a lower press die 53 is fixed to a bolster plate 54. The bolster plate 54 is supported by a carrier (corresponding to a press carrier 55 of Patent Document 1). A blank holder (corresponding to a cushion pad of Patent Document 1) 56 is disposed within the lower press die 53. The blank holder 56 is supported by the top ends of plural cushion pins 57, and the bottom ends of the individual cushion pins 57 are supported by a cushion plate 69 of a die cushion 59 through hydraulic cylinders 58. The die cushion 59 is provided with a cushion cylinder 61 for supporting the cushion plate 69, and a prescribed air pressure, which is determined by an air regulator 63, is supplied from an air pressure source 62 to the cushion cylinder 61 via an air tank 64. The individual hydraulic cylinders 58 are connected to an oil feeding means 67 through a common pipe 65 and a flexible tube 66. The oil feeding means 67 has a function to assure an amount of oil being supplied to the individual hydraulic cylinders 58.
According to the above configuration, a work W is placed on the blank holder 56, the slide 52 is lowered, its lowering force is transmitted to the die cushion 59 through the cushion pins 57, and the die cushion 59 generates a cushion pressure, namely a blank holding force. At this time, an oil pressure generated within each of the hydraulic cylinders 58 disposed at the bottom ends of the cushion pins 57 becomes constant because the individual hydraulic cylinders 58 are mutually communicated. Thus, unbalanced cushion pressures of the individual cushion pins 57 are smoothed (namely, equalized) to improve blank holding accuracy.
But, the above-described conventional cushion pin pressure-equalizing device has the following drawbacks. Specifically, to smooth the unbalanced cushion pressures among the cushion pins 57, it is necessary to provide an equalizing hydraulic circuit, which has the hydraulic cylinders 58 disposed for the individual cushion pins 57 connected to the oil feeding means 67 through the common pipe 65 or an oil passage. Thus, the cushion pin pressure-equalizing device has disadvantages that it becomes large in scale and its cost is high. Besides, a press production site generally has a press machine, which is used for trials to determine pressing conditions in advance before real production. This press machine is in addition to the press machine that is used for the real production. This tryout press machine is also provided with the above-described cushion pin pressure-equalizing device, so that the cost is doubled, and the high cost problem cannot be neglected.
Besides, for pressing with high accuracy, it may be necessary to vary the blank holding force for each portion depending on the shape even if the work is the same. For example, areas D1 to D4 are provided by dividing the portion around the center of the die into four areas as shown in FIG. 16, and four die cushions 59 with different cushion pressures for the individual areas D1 to D4 and the cushion pin pressure equalizing device corresponding to the individual die cushions 59 are provided. Therefore, the cost becomes higher.
The present invention has been made in view of the above circumstances and provides equalization of the cushions without using a large-scale hydraulic circuit and results in reduction of a press machine production cost.